Waveguide antenna

ABSTRACT

This invention relates to a multi-frequency phased array steerable beam waveguide antenna in which the waveguide radiator elements have a cut-off frequency between two operating frequencies of the antenna. At the lower frequency the waveguides are terminated to function as evanescent mode resonators giving a first passband centered on the lower frequency. At the higher frequency the waveguides are propagating and are terminated to give a second passband centered on the higher frequency.

Corporation, New York, N.Y.

United States Patent [191 [111 3,825,932 Hockham [4 July 23, 1974 [54] WAVEGUIDE ANTENNA 3,761,937 9/1973 Tricolesetal. 343/770 [75] Inventor: George Alfred Hockham,'Takeley,

England Prirnary ExaminerJames W. Lawrence [73] Assignee: International Standard Electric Assstam Exammer"'Mar"m Nussbaum Attorney, Agent, or Firm-John T. OHalloran; Menotti J. Lombardi, Jr.; Vincent lngrassia [22] Filed: May 16, 1973 [21] Appl. No.: 360,724

[57] ABSTRACT [30] Forelgn Applicafim! Priority Data This invention relates to a multi-frequency phased June 8, 1972 Great Br1ta1n... 26760/72 array t erable bearn waveguide antenna in which the waveguide radiator elements have a cut-off frequency [52] US. Cl 343/776, 343/784, 343/789, between two Operating frequencies of the antenna At 343/858 343/862 the lower frequency the waveguides are terminated to [51] I Cl 13/06 Holq 1/485 Holq 5/00 function as evanescent mode resonators giving a first [58] Field of Search.....- 343/ 854, 783-784, passband centered on the lower frequency At the 343/786, 776, 777, 772, 770, 789, 858, 862 higher frequency the waveguides are propagating and are terminated to ive a second assband centered on [56] References d the higher frequeniy. UNITED STATES PATENTS 2,658,145 11/1953 Dorne et al 343/772 3 Claims, 3 Drawing Figures 6 9A a {0A Z; [j U L|.

I l .6 A -Z5 7 Fl [1 PATENTED 3.825.932

SHEET 20F 2 WAVEGUIDE ANTENNA BACKGROUND OF THE INVENTION This invention relates to waveguide antennas, and particularly but not exclusively to phased array steerable beam antennas.

When such steerable beam antennas are used, for example, in shipborne radar equipement, the array may consist possibly of several thousand individual equipment, radiators. There may be a number of such equipments on one ship, with each equipment having its antenna designed for operation at the particular frequency of operation of that equipment.

It is known that a phased array steerable beam antenna may be constructed using as the individual radiator elements open-ended waveguides operating in the evanescent mode. Since such evanescent waveguide has a significantly smaller size for a given operating frequency than propagating waveguide, this evanescent waveguide antenna offers the advantage of a reduction in both space requirements and in weight as compared with propagating waveguide, 'and also facilitates achieving a suitable spacing between adjacent waveguides so as to obtain a practically useful beam scan capability (of up to i60) while limiting secondary beams in real space.

SUMMARY OF THE INVENTION It is an object of the present invention to minimize the number of different antenna arrays, for example on space-craft or ships, resulting in significant overall cost, weight and size reductions.

According to a broad aspect of the invention there is provided a multiple frequency waveguide antenna comprising a plurality of radiating elements each comprising a length of open-ended waveguide having a cutoff frequency between first and second operating frequencies of the antenna; and means for terminating each of said plurality so that the radiating element has a first passband centered on the first operating frequency and a second passband centered on the second operating frequency.

The above and other objects of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a radiating end view of a dual frequency phase array steerable beam antenna;

FIG. 2 is a side view showing details of a waveguide radiator element with a single input; and

FIG. 3 is a side view showing details of a waveguide radiator element with dual inputs.

DESCRIPTION OF THE PREFERRED EMBODIMENT The antenna shown in FIG. 1 is designed for either separate or simultaneous operation at two different frequencies, e.g., 1.3 GHz (L-Band) and 3.6 GHz (S- band), and comprises an array of waveguides l terminating open-ended at apertures 2 in a conducting ground plane 3.

As shown in FIG. 1, the array is in the form of staggered rows (a triangular lattice configuration). The row pitch is 0.94 M, where A is the wavelength at the higher operating frequency and the column pitch is 0.33 A Each waveguide is of rectangular cross-section with a width of 0.91 A

As shown in FIG. 2 terminating each waveguide, of 5 length d), is a slice of dielectric 4 with an inductive iris as the aperture, and there is a single input 5. A

The width of each waveguide is such that for the higher operating frequency, it is propagating. With the approximately threefold increase in wavelength at the lower operating frequency, the waveguides are nonpropagating at the lower operating frequency, i.e., the cut-off frequency is between the two operating frequencies.

At the lower operating frequency, the length d) and the termination provided by the dielectric slice 4 are so chosen that the waveguide functions as an evanescent mode resonator so that there is a passband centered on this frequency.

The mechanism by which there is complete energy transfer through evanescent waveguides is fully described in Waveguide Bandpass filters Using Evanescent Modes M. F. Craven, Electronics Letters, Vol. 2, No. 7, July 1966, pp 25-26, and in US. Pat. No. 3,621,483, entitled Waveguide Filter, issued Nov. 16, 1971 and assigned to the same assignee as the instant invention.

However, the mechanism may be briefly stated as in the following paragraph.

As is well known, dominant mode waveguide'ceases to propagate progressive waves below its cut-off frequency, and the mode is said to be evanescenL'Waveguide in which the dominant mode is evanescent has a positive imaginary (inductive) characteristic impedance (jZ to an incident H mode and a real propagation constant (1 and therefore behaves essentially as a pure reactance. If a short section (of length qb) of this guide is terminated in an obstacle which presents a conjugate (capacitive) reactance at a frequency below the cut-off frequency, the incident power at that frequency will be completely transmitted through the section.

The evanescent mode resonator formed by the waveguide at the lower operating frequency of the antenna therefore comprises a single section resonator (of length (1)), the conjugate match being provided by the dielectric slice as the capacitive obstacle. This dielectric at the open end of the waveguide serves to provide both physical continuation of the ground plane and also to take into account the junction effect at the aperture.

At the higher operating frequency the waveguides are propagating. Accordingly the dielectric slice at this higher frequency is additionally arranged to match the waveguide impedance at this frequency, so that there is a passband centered on this higher frequency.

The resulting beam radiated by the array is steered in known manner by suitable control of phase shifters (not shown) in the feed circuit to the inputs of the waveguides from a microwave source or sources supplying the operating frequencies.

Alternatively, and preferrably, as shown in FIG. 3, each waveguide possesses a separate input for each band of operation, input 6 for L-band and input 7 for S-band, with a metal septum 8 (E-plane bifurcation) so that the structure consists of two reduced height waveguides located on top of one another for the length of the bifurcation and then a single waveguide terminated in a dielectric plug 4. The use of separate connectors 3. improves the isolation between the ports which is important for the successful operation of a practical system.

In reach reduced height waveguide there is a tuning screw 9A, 9B respectively, and matching obstacles A, 103 respectively. The distance between obstacle 10A and the end of the bifurcation 8 is (1) and the distance between the end of the bifurcation 8 and the dielectric plug, of thickness (1) is The aperture dimensions (iris width) dielectric thickness-and permittivity are chosen for the antenna (a) to resonate at the desired L-band frequency since the unloaded waveguide is below cut-off for this frequency and (b) for a match to be achieved at the desired S-band frequencies.

The L-band section of the antenna is a three cavity structure consisting of the coupling loop, intermediate obstacle 10A and the aperture. In this manner the lowest VSWR over the operating band is achieved, and because obstacle 10A must be quite a large susceptance it presents a good obstacle to the S-band energy coupled round the edge of the bifurcation and consequently gives rise to good isolation. Coupling at L-band to the high frequency port is low because the waveguide is operating below cut-off at this frequency and since it is untuned the field decays quite rapidly away from the bifurcated edge towards the S-band coupling loop. The performance of the L-band section of the antenna is also dependent on the length but is quite insensitive to the actual value of (b provided the obstacle 10A does not become too close to the bifurcated edge. This is important since the length can be chosen to match the aperture at the high frequency band. 7

It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation on its scope.

What is claimed is:

l. A multiple frequency waveguide antenna comprising:

a plurality of radiating elements each comprising a length of openended waveguide having a cut-off frequency between first and second operating frequencies of the antenna, wherein each of said plurality has aseparate input for each of said operating frequencies and wherein each of said plurality has an E-plane bifurcation beginning at said sepa-' rate inputs and extending along a partial length of said element; and

means for'terminating each of said plurality so that the radiating element has a first passband centered on the first operating frequency and a second passband centered on the second operating frequency.

2. A waveguide antenna according to claim 1, wherein each of said plurality terminates open-ended at an aperture in a conducting ground plane.

3. A waveguide antenna according to claim 2, wherein a slice of dielectric material is provided at said aperture. 

1. A multiple frequency waveguide antenna comprising: a plurality of radiating elements each comprising a length of openended waveguide having a cut-off frequency between first and second operating frequencies of the antenna, wherein each of said plurality has a separate input for each of said operating frequencies and wherein each of said plurality has an E-plane bifurcation beginning at said separate inputs and extending along a partial length of said element; and means for terminating each of said plurality so that the radiating element has a first passband centered on the first operating frequency and a second passband centered on the second operating frequency.
 2. A waveguide antenna according to claim 1, wherein each of said plurality terminates open-ended at an aperture in a conducting ground plane.
 3. A waveguide antenna according to claim 2, wherein a slice of dielectric material is provided at said aperture. 